backing-file: use credential guards for reads

Use credential guards for scoped credential override with automatic
restoration on scope exit.

Link: https://patch.msgid.link/20251103-work-creds-guards-simple-v1-3-a3e156839e7f@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/backing-file.c b/fs/backing-file.c
index 15a7f80310848da3ed5801efa90cef243f0672ec..37f3bf29595f5a319f13139a9a092d1110833df3 100644 (file)
--- a/fs/backing-file.c
+++ b/fs/backing-file.c
@@ -157,13 +157,37 @@ static int backing_aio_init_wq(struct kiocb *iocb)
        return sb_init_dio_done_wq(sb);
 }
 
+static int do_backing_file_read_iter(struct file *file, struct iov_iter *iter,
+                                    struct kiocb *iocb, int flags)
+{
+       struct backing_aio *aio = NULL;
+       int ret;
+
+       if (is_sync_kiocb(iocb)) {
+               rwf_t rwf = iocb_to_rw_flags(flags);
+
+               return vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
+       }
+
+       aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+       if (!aio)
+               return -ENOMEM;
+
+       aio->orig_iocb = iocb;
+       kiocb_clone(&aio->iocb, iocb, get_file(file));
+       aio->iocb.ki_complete = backing_aio_rw_complete;
+       refcount_set(&aio->ref, 2);
+       ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
+       backing_aio_put(aio);
+       if (ret != -EIOCBQUEUED)
+               backing_aio_cleanup(aio, ret);
+       return ret;
+}
 
 ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
                               struct kiocb *iocb, int flags,
                               struct backing_file_ctx *ctx)
 {
-       struct backing_aio *aio = NULL;
-       const struct cred *old_cred;
        ssize_t ret;
 
        if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -176,28 +200,8 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
            !(file->f_mode & FMODE_CAN_ODIRECT))
                return -EINVAL;
 
-       old_cred = override_creds(ctx->cred);
-       if (is_sync_kiocb(iocb)) {
-               rwf_t rwf = iocb_to_rw_flags(flags);
-
-               ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
-       } else {
-               ret = -ENOMEM;
-               aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
-               if (!aio)
-                       goto out;
-
-               aio->orig_iocb = iocb;
-               kiocb_clone(&aio->iocb, iocb, get_file(file));
-               aio->iocb.ki_complete = backing_aio_rw_complete;
-               refcount_set(&aio->ref, 2);
-               ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
-               backing_aio_put(aio);
-               if (ret != -EIOCBQUEUED)
-                       backing_aio_cleanup(aio, ret);
-       }
-out:
-       revert_creds(old_cred);
+       scoped_with_creds(ctx->cred)
+               ret = do_backing_file_read_iter(file, iter, iocb, flags);
 
        if (ctx->accessed)
                ctx->accessed(iocb->ki_filp);